There has been known and used in the explosive industry, slurry explosives which are safer to produce and handle than high explosives. These slurry explosives require chemical or explosive sensitizers to assure detonation and some incorporated gas bubbles.
Subsequently developed were water-in-oil emulsion explosives wherein a discontinuous phase of aqueous solution of inorganic oxidizer salt was dispersed in a continuous phase of carbonaceous fuel. See U.S. Pat. No. 3,447,978.
The above described slurry explosives use chemical or explosive sensitizers, such as monomethylamine nitrate, ethyleneglycol mononitrate, ethanolamine mononitrate, ethylenediamine mononitrate, aluminum powder, PETN, TNT and smokeless powder in order to retain explosive performance. However, the water-in-oil emulsion explosive does not require the use of a chemical or explosive sensitizer. However, the water-in-oil emulsion requires uniformly dispersed void spaces provided by gas bubbles or a void-providing agent to obtain explosive performance. Therefore, maintaining the uniformly dispersed void spaces in the water-in-oil emulsion explosive is important in achieving good detonation performance and good shelf life. Furthermore, the manner in which void spaces are treated may effect the explosive properties of the emulsion explosive.
Void spaces can be provided by gas bubbles which are mechanically or physically mixed or blown into an emulsion explosive. Voids can also be formed in an emulsion explosive by a chemical gassing agent, or mixed into an emulsion explosive by a void-providing agent, such as hollow microspheres, expanded perlite or styrofoam beads. The use of gas bubbles or gassing agents is less desirable because bubbles leak and coalesce during the storage of the explosive and thus decrease the detonation sensitivity. Another disadvantage is that under hydrostatic pressure the gas bubbles provide less effective density control which affects detonation sensitivity and performance.
Use of density control agents such as expanded perlite have been known in the emulsion explosive industry for many years. For example, such density control agents are described in Wade's U.S. Pat. No. 3,715,247. Additionally, patents exist on use of specific sizes of perlites such as preferred particle size ranges when used in explosive compositions, e.g., Sudweeks, et al., U.S. Pat. No. 4,231,821. The widespread current commercial practice is to use hollow glass microspheres (e.g., as provided by 3M or PQ) for emulsions which will be stored and/or handled in a bulk form, e.g., multiple pumping. Glass microspheres are different from perlites. A major difference is that the perlites are by nature porous, whereas hollow glass microspheres are non-porous. This difference in physical structure has caused the use of perlites to be limited. This is because a porous particle cannot maintain adequate density control of the final emulsion explosive product over time. In particular, pumping and other forms of applied work/pressure will cause irreversible density rise, leading to reduced performance of the explosive composition. Thus, it is desirable to use a density control agent which is not adversely affected by typical product handling, such as, shipping and pumping, or product application, i.e. use in wet boreholes where hydrostatic head pressure exist. Furthermore, with present-day perlites, density control over long periods of storage is not possible. With conventional perlites, density control is not maintained when the explosive product is subjected to agitation or shaking resulting from over-the-road handling of the product in bulk containers where this vibration and movement of the emulsion explosive can impart work on it. Where conventional perlites are used, density rise is found and continues to worsen with time. Furthermore, there has previously been an unacceptable viscosity rise which adversely affects handling, specifically pumping.
An expanded perlite has been discovered which imparts desirable features and technical advantages to emulsion explosives. Specific desirable features which the expanded perlite imparts on emulsion explosive compositions are as follows: (1) density control is maintained following multiple pumping of hot or cold emulsions containing the new perlites; (2) density control is maintained following exposure of the emulsion containing the new perlites to over-the-road handling with minimum viscosity rise of product during test; (3) detonation performance is attained in 4 inch diameter borehole when exposed to hydrostatic pressure using unpumped or pumped expanded perlite samples with detonation velocities in the cartridge-to-cartridge pressure bomb test of at least 6,000 ft/sec and preferably 15,000-16,000 ft/sec; and (4) detonation performance is attained in 3 inch diameter borehole at ambient pressure with reduced temperature when carried out on unpumped or on multiple pumped explosive products with detonation velocity in the cartridge-to-cartridge test of at least 6,000 ft/sec and preferably 15,000-16,000 ft/sec.